In recent years, with higher integration of semiconductor devices, fine patterns exceeding the transfer limit of a photolithography method using the conventional ultraviolet light have been required in the semiconductor industry. In order to enable transfer of such fine patterns, the EUV lithography being an exposure technique using extreme ultraviolet light (hereinafter referred to as EUV light) with a shorter wavelength is expected to be promising. The EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 nm to 100 nm. As an exposure mask for use in the EUV lithography, there is proposed a reflective mask as described in Patent Document 1.
This reflective mask has a structure in which a multilayer reflective film for reflecting the EUV light serving as exposure light is formed on a substrate and, further, an absorber film for absorbing the EUV light is formed in a pattern on the multilayer reflective film. When pattern transfer is carried out using an exposure apparatus (pattern transfer apparatus) with the reflective mask disposed therein, the exposure light incident on the reflective mask is absorbed at a portion where the absorber film pattern is present, but is reflected by the multilayer reflective film at a portion where the absorber film pattern is not present. The light reflected by the multilayer reflective film is irradiated onto, for example, a semiconductor substrate (resist-coated silicon wafer) through a reflective optical system so that the absorber film pattern of the reflective mask is transferred.
Apart from the reduction in wavelength of light, a resolution improvement technique using a phase shift mask is proposed by Levenson et al. of IBM (e.g. Patent Document 2). In the phase shift mask, a transmitting portion of a mask pattern is made of a substance or has a shape which is different from that of its adjacent transmitting portion so that a phase difference of 180 degrees is given to lights transmitted therethrough. Therefore, in an area between both transmitting portions, the transmitted diffracted lights different in phase by 180 degrees cancel each other out and thus the light intensity becomes extremely small to improve the mask contrast. As a result, the depth of focus upon transfer increases to improve the transfer accuracy. The phase difference is theoretically best at 180 degrees, but if it is substantially about 175 to 185 degrees, a resolution improvement effect is obtained.
A halftone mask which is one type of a phase shift mask is a phase shift mask that improves the resolution of a pattern edge portion by using a light-absorbing thin film as a material of a mask pattern to give a phase difference of 180 degrees with respect to normal light transmitted through a substrate while reducing the transmittance to about several % (normally about 5% to 20%). As a light source, it is currently shifting from a KrF excimer laser (wavelength 248 nm) hitherto used to an ArF excimer laser (wavelength 193 nm).
However, even with the ArF excimer laser, in terms of additional problems in exposure apparatus and resist, the lithography technique using the halftone mask is not easily applied as a futuristic lithography technique for manufacturing a device with a line width of 50 nm or less.
In view of this, in order to further improve the transfer resolution of the EUV lithography, there is proposed an EUV exposure mask that makes it possible to apply the theory of a halftone mask used in conventional excimer laser exposure or the like to the EUV lithography using a reflective optical system (e.g. Patent Document 3).